Variable heart rate decelerations and transcutaneous Po2 during umbilical cord occlusion in fetal monkeys

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      In five near-term rhesus monkeys under ketamine anesthesia, fetal heart rate, blood pressure, intrauterine pressure, and transcutaneous oxygen tension (tcPo2) were recorded continuously while the umbilical cord was occluded for 15 or 30 seconds. Fetal heart rate decreased 55 ± 19 bpm, (mean ± SD) and 78 ± 20 bpm, respectively, with 15 and 30 second occlusions, while tcPo2 declined 6.0 ± 1.6 and 11.5 ± 3.3 torr, respectively. Fetal hypertension started promptly with cord occlusion and reached a maximum within 5 to 16 seconds; fetal blood pressure decreased thereafter and, in most instances, fell below the baseline until fetal heart rate began to recover. Fetal heart rate decreased within 1 to 2 seconds of cord occlusion. tcPo2 changes were the same, while heart rate changes were attenuated when the fetus was pretreated with 0.2 mg of atropine. These findings support a rapid and marked initial baroreceptor response, followed by a less marked chemoreceptor response. Thirty seconds following cord occlusion, Pao2 had returned to baseline, white tcPo2 was still 3 to 7 torr below baseline, indicating cutaneous vasoconstriction even though systemic blood pressure was normal. Relative heat output required to maintain the tcPo2 electrode temperature at 44°C did not reflect alterations in skin perfusion during periods of cardiovascular change.
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